An imaging system of the kind set forth is known from European Patent Application EP 295,728, now U.S. Pat. No. 4,857,724.
According to the cited patent, a circular exit screen of an image intensifier tube is imaged, via the optical system, as an inscribed ellipse on a rectangular CCD matrix, the long and short axes of the ellipse being equal to the long side and the short side, respectively, of the CCD matrix. Thus, optimum use is made of the detection face of the CCD matrix and the resolution is enhanced in the direction of the long axis of the ellipse. The ellipse can be displayed as a circle again on a display unit by accelerated reading of the detection sub-faces of the CCD matrix in a row direction (video line direction). In addition to an efficient as possible use of the detection face of the image sensor, it is desirable to adapt the number of active detection sub-faces to the display means used which comprise, for example a television picture tube or a digital data processing or data storage device.
When the display means comprise a cathode ray tube, the signals of the detection sub-faces are made visible on a phosphorescent display screen along the image lines. To this end, the charge accumulated, for example, underneath the detection sub-faces in a CCD image sensor of the frame transfer type, is introduced into a shift register in parallel per row of detection sub-faces, the charge being applied from said shift register to the cathode ray tube per detection sub-face. The signal value of a detection sub-face modulates the intensity of an electron beam scanning the display screen and becomes visible as a brightness variation with respect to a dark level on the display screen. The detection sub-faces are thus distributed as if it were across the display screen, the position of the detection sub-faces along an image line and the position of a row of detection sub-faces on the display screen being determined by the frequency at which the detection sub-faces are shifted out of the shift register. The number of rows of detection sub-faces of the image sensor which can be displayed on the display screen is determined by the number of lines constituting the image on the display screen. When the total number of rows of detection sub-faces of the image sensor amounts to n and the number of lines of the display screen of the display means amounts to 2, where n&gt;z, not all rows of detection sub-faces can be displayed on the display screen. As a result, image compression of a round image on the image sensor into an elliptical image which covers the surface of the image sensor as well as possible, is of limited use. In such a case the optical system distorts a round image by a compression factor m, m being the ratio of the short axis of the ellipse to the long axis of the ellipse, where m=b/a. Therein, b is the short side of the rectangular detection face and a is the long side, thereof.